the connectionString property has not been initialized

[athir13en]

i face a problem that show connectionString not yet been initialize..
in web.config i hav these code

    <connectionStrings>
        <add name="ConnectionString" connectionString="Provider=SQLOLEDB;Data Source=SERVERNAME;Persist Security Info=True;Password=PASSWORD;User ID=USER;Initial Catalog=Northwind"
            providerName="System.Data.OleDb" />
    </connectionStrings>

and in my .vb page i hav these

Protected Sub btnAddUser_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnInsertCategory.Click
        Dim sqlConn As New SqlConnection
        Dim strConnection As String

        strConnection = System.Configuration.ConfigurationManager.AppSettings("connectionString")
        sqlConn = New SqlConnection(strConnection)
        sqlConn.Open()
        Try
            Dim command As New SqlCommand("spAddUser", sqlConn)
            command.CommandType = CommandType.StoredProcedure

            command.Parameters.AddWithValue("@intUserID", "6")
            command.Parameters.AddWithValue("@ncharname", "Pauly")
            command.Parameters.AddWithValue("@ncharUserName", "Paul")
            command.Parameters.AddWithValue("@intDepartmentID", "1")
            command.Parameters.AddWithValue("@intContactNo", "1235")

            command.ExecuteNonQuery()
        Catch ex As Exception
            MsgBox(Err.Description, MsgBoxStyle.Critical)
            Throw
        Finally
            sqlConn.Close()
        End Try
    End Sub

pls help... thanks..